1. Field of the Invention
This invention relates to electronic motor braking systems, and more particularly, to an electronic braking system having means for quickly applying a DC braking current to an AC motor when the running current is terminated.
2. Description of the Prior Art
Electronic or dynamic braking for slowing down and stopping electrical AC motors is well known in the motor art. It offers the advantage of being smooth, it provides accurately timed stops and is not subject to the severe problems of rubbing surfaces used in mechanical braking systems. Dynamic braking employs the principle of removing an alternating current from the motor and applying an amplitude and time controlled DC current to the stator windings of the AC motor. The DC current produces a static field across the stator which generates a counterforce in the rotor of the motor to rapidly decelerate the motor without any mechanical braking. Preferably this dynamic braking is achieved without any modification of the motor or a parallel connected motor brake AC solenoid. A mechanical brake is employed to hold the motor in a stopped position and may also be used to help bring the motor to a complete stop.
Some of the prior art dynamic braking circuits employ a timing circuit which is connected between two of the AC power lines which are connected to the AC motor. A brake release coil is connected across the same lines and when energized the coil holds a spring loaded mechanical brake mounted on the motor in its released position. As long as power is uninterruptedly connected to the aforesaid motor lines the brake release coil disengages the mechanical brake from the motor. After the timer has completed its timing sequence, the release coil is de-energized and the mechanical brake is engaged by the brake spring.
Unfortunately, a slight interruption in the supply of current to the brake release coil (such as the interruption which can occur during changeover from AC power to electric braking) causes the mechanical brake to be prematurely applied to the motor. This causes brake heating and fade if the brake is engaged at a high motor speed. What is needed is a timed DC braking circuit which applies dynamic braking current when the motor is disconnected from the AC power source so quickly that if a mechanical brake coil is connected across the motor lines, the brake coil will remain sufficiently energized to hold the mechanical brake in its released position for a predetermined amount of dynamic braking time, and does not apply any mechanical braking in the event of a failure of the braking circuit or its timer while the motor is running under power.